Rumsfeld last Friday passed Caspar Weinberger, to take over the No. 2 spot on the list. In December he'll have served longer than McNamara, USA Today reports.
Working for both U.S. Presidents John Kennedy and Lyndon Johnson, McNamara retired in early 1968, after serving since 1961.
Rumsfeld, 74, already has the record as both the youngest and oldest person to head up the Defense Department after marking his first tour of duty during the Ford administration. Rumsfeld was secretary of Defense from 1975-77 and took the position again upon U.S. President George Bush's first inauguration on Jan. 20, 2001.
Longevity aside, Rumsfeld is said to be the policymaker who gets most of the heat for U.S. missteps in Iraq.
In "Hard Power: The New Politics of National Security" being published next month, co-author Michael O'Hanlon predicts Rumsfeld will follow in the footsteps of McNamara who was blamed for the Vietnam conflict.
In a USA TODAY/Gallup poll taken Sept, 15-17, 46 percent of those asked said Rumsfeld should resign while 44 percent said he shouldn't.
Whenever he's asked about stepping down, Rumsfeld always says he serves at the pleasure of the president.